1. Field of the Invention
The present invention relates to a laser driver circuit which drives a laser diode, and a laser display which comprises the laser driver circuit.
2. Description of the Related Art
Conventionally, there is known a laser display which scans a laser beam across a screen in order to display images on the screen. The laser display comprises a laser driver circuit for driving a laser diode. The laser display drives a laser diode with the laser driver circuit so as to irradiate the screen with the laser beam. FIG. 5 illustrates an example of a conventional laser driver circuit.
A laser driver circuit 100 shown in FIG. 5 is a current drive type laser driver circuit which drives a laser diode 101 by supplying the laser diode 101 with a current. In order to drive the laser diode 101, the laser driver circuit 100 supplies the laser diode 101 with not only a threshold current from a current supply circuit 140-0 for supplying the threshold current but also a radiance control current from plural current supply circuits 140-1 to 140-8 for supplying the radiance control current. The above-described threshold current is defined as the minimum current necessary to make the laser diode 101 emit light. In other words, the threshold current is a current corresponding to a threshold voltage. The above-described radiance control current is defined as a current for adjusting radiance (or radiant intensity) of the laser diode 101.
In the current supply circuit 140-0 for supplying the threshold current, a threshold voltage output from a threshold voltage output circuit (T-voltage output) 147-0 is input to a voltage-to-current converter circuit 141-0 (V/C converter) via a voltage input line 146-0 so that the voltage-to-current converter circuit 141-0 generates the threshold current corresponding to the input threshold voltage. The generated threshold current is output via a current output supply line 142-0. The threshold current output via the current output supply line 142-0 is supplied to the laser diode 101.
In the current supply circuit 140-1 for supplying the radiance control current, a radiance voltage output circuit (R-voltage output) 147-1 generates a radiance control voltage based on a voltage supplied from a maximum voltage supply source 150 so as to input the radiance control voltage to a voltage-to-current converter circuit (V/C converter) 141-1 via a voltage input line 146-1. The voltage-to-current converter circuit 141-1 generates the radiance control current corresponding to the input radiance control voltage so as to output the radiance control current to a current output line 142-1. When the current output line 142-1 and a current supply line 143-1 are connected by a current path switch 145-1, the radiance control current running through the current output line 142-1 is supplied to the laser diode 101 via the current supply line 143-1. On the other hand, when the current output line 142-1 and an ground line 144-1 are connected by the current path switch 145-1, the radiance control current running through the current output line 142-1 flows to the ground via the ground line 144-1.
Similarly, in each of the current supply circuits 140-2 to 140-8 for supplying the radiance control currents, each of radiance voltage output circuits (R-voltage outputs) 147-2 to 147-8 generates each radiance control voltage based on the voltage supplied from the maximum voltage supply source 150 so as to input the each radiance control voltage to each of voltage-to-current converter circuits 141-2 to 141-8 (V/C converters) via each of voltage input lines 146-2 to 146-8. Each of the voltage-to-current converter circuits 141-2 to 141-8 generates each radiance control current corresponding to the input each radiance control voltage so as to output the each radiance control current to each of current output lines 142-2 to 142-8.
When each of the current output lines 142-2 to 142-8 and each of current supply lines 143-2 to 143-8 are connected by each of current path switches 145-2 to 145-8, the each radiance control current running through each of the current output lines 142-2 to 142-8 is supplied to the laser diode 101 via each of the current supply lines 143-2 to 143-8. On the other hand, when each of the current output lines 142-2 to 142-8 and each of ground lines 144-2 to 144-8 are connected by each of the current path switches 145-2 to 145-8, the each radiance control current running through each of the current output lines 142-1 to 142-8 flows to the ground via each of the ground lines 144-2 to 144-8.
Each radiance control voltage to be output by each of the radiance voltage output circuits 147-1 to 147-8 in each of the current supply circuits 140-1 to 140-8 is different from each other. Accordingly, each of the voltage-to-current converter circuits (V/C converters) 141-1 to 141-8 generates the radiance control current with each different current value.
The current output supply line 142-0 (in the current supply circuit 140-0) and each of the current supply lines 143-1 to 143-8 (in each of the current supply circuits 140-1 to 140-8) are connected in parallel. Accordingly, both the threshold current running through the current output supply line 142-0 and the radiance control currents running through the current supply lines 143-1 to 143-8 are converged (added) and supplied to the laser diode 101 in order to drive the laser diode 101.
Whether each of the current output lines 142-1 to 142-8 is connected to each of the current supply lines 143-1 to 143-8 or each of the ground lines 144-1 to 144-8 is switched by the current path switches 145-1 to 145-8. Thus, a value of the current supplied to the laser diode 101 can be changed, so that the radiance of the laser beam to be emitted by the laser diode 101 can be changed. The above-described switching using the current path switches 145-1 to 145-8 is controlled by a switch control circuit 170.
The switch control circuit 170 controls the current path switches 145-1 to 145-8 so as to control switching whether each of the current output lines 142-1 to 142-8 is connected to each of the current supply lines 143-1 to 143-8 or each of the ground lines 144-1 to 144-8. Thus, a value of the current supplied to the laser diode 101 can be controlled, so that the radiance of the laser beam to be emitted by the laser diode 101 can be controlled.
However, in order to enable the radiance of the laser beam to be emitted by the laser diode 101 to be rapidly controlled, the conventional laser driver circuit 100 cuts the radiance control current(s) to the laser diode 101 without halting voltage-to-current convert function of the voltage-to-current converter circuit(s) in current supply circuit(s), which do (does) not supply the radiance control current(s), out of the current supply circuits 140-1 to 140-8. More specially, in the current supply circuit(s) which do (does) not supply the radiance control current(s), the conventional laser driver circuit 100 cuts the radiance control current(s) to the laser diode 101 by connecting the current output line(s) to the ground line(s). In other words, in the current supply circuit(s) which do (does) not supply the radiance control current(s), the conventional laser driver circuit 100 keeps not only generating the radiance control current(s) using the voltage-to-current converter circuit(s), but also sending the generated radiance control current(s) to the ground so as not to supply the laser diode 101 with the radiance control current(s). Thus, the conventional laser driver circuit 100 generates and outputs wasteful currents which do not contribute to emission of the laser diode 101, and thereby requires a large amount of power.